The present invention provides a process for producing a membrane electrode assembly (MEA) for fuel cells, which is especially suitable for continuous manufacture of membrane electrode assemblies.
A membrane electrode assembly consists of a polymer electrolyte membrane, both faces of which are each provided with a catalyst layer and a gas distribution layer arranged on top of the catalyst layer. One of the catalyst layers is designed as an anode for the oxidation of hydrogen and the second catalyst layer is designed as a cathode for the reduction of oxygen. The gas distribution layers normally consist of carbon fiber paper or carbon fiber fabric and enable good access by the reaction gases to the reaction layers and good conductance of the cell current. The catalyst layers for anode and cathode contain a proton-conducting polymer and so-called electrocatalysts which catalytically support the relevant reaction (oxidation of hydrogen and reduction of oxygen). Metals from the platinum group in the Periodic System of Elements are preferably used as catalytically active components. In the majority of cases so-called supported catalysts are used in which the catalytically active platinum group metals have been applied in highly dispersed form to the surface of a conductive support material. Finely divided carbon blacks have proved useful as support materials.
The polymer electrolyte membrane consists of proton-conducting polymer materials. These materials are also called ionomers for short in the following. A tetrafluoroethylene/fluorovinylether copolymer with sulfonic acid groups is preferably used. This material is marketed, for example, by DuPont under the trade name Nafion®. However, other, in particular fluorine-free, ionomer materials such as sulfonated polyetherketones or arylketones or polybenzimidazoles can also be used. For use in fuel cells, these membranes generally have a thickness between 10 and 200 μm.
The catalyst layers are mostly applied to the polymer electrolyte membranes using a pasty preparation by printing, spreading, rolling or spraying. The pasty preparations are called inks or catalyst inks in the following. In addition to the supported catalyst, they generally contain a soluble proton-conducting material, several solvents and optionally highly dispersed hydrophobic materials and pore-forming agents. Catalyst inks can be differentiated by the type of solvent used. There are inks which contain predominantly organic solvents and those which use predominantly water as the solvent. Thus DE 196 11 510 A1 describes catalyst inks which contain predominantly organic solvents, while EP 0 731 520 A1 describes catalyst inks in which exclusively water is used as the solvent.
The gas distribution layers usually consist of coarse-pored carbon fiber paper or carbon fiber fabric with a porosity of up to 90%. In order to prevent flooding of the pore system with the reaction water being produced at the cathode, these materials are impregnated, for example, with dispersions of polytetrafluoroethylene (PTFE). Calcination at about 340 to 370° C. follows impregnation in order to melt the PTFE material. To improve electrical contact between the catalyst layers and the gas distribution layers, these are often coated, on the surface turned towards the relevant catalyst layer, with a microporous layer consisting of carbon black and a fluorinated polymer, which is porous and water-repellent and at the same time electrically conductive, and in addition has a reasonably smooth surface.
To use fuel cells as sources of electrical energy, many membrane electrode assemblies are arranged on top of each other to form a fuel cell stack. So-called bipolar sheets are introduced in between the individual membrane electrode assemblies and these lead the reaction gases to the electrodes in the fuel cell and lead the reaction products away via corresponding channels. In addition they take on the task of supplying and removing the cell current.
The use of these fuel cell stacks for electrical drive units in motor vehicles requires large-scale production processes for the membrane electrode assemblies.
DE 195 09 749 A1 describes a process for continuous production of a composite of electrode material, catalyst material and a solid electrolyte membrane, wherein a catalyst powder comprising the electrode material, the catalyst material and the solid electrolyte material is used to form a catalytic coating on a carrier. This catalyst layer is heated to soften the solid electrolyte material and rolled out under pressure on the solid electrolyte membrane. This procedure is performed for both faces of the solid electrolyte membrane so that the process provides a complete membrane electrode assembly. The carrier for the catalyst layer acts as a gas distribution layer in the final membrane electrode assembly.
WO 97/50142 describes a continuous process for coating a polymer electrolyte membrane with electrodes, in which a strip-shaped polymer membrane is drawn through a bath of a platinum salt solution. The adhering salt is then reduced to the noble metal in a gas stream or in another bath. This process does not provide complete membrane electrode assemblies.
WO 97/23916 also describes a process for continuous production of material composites, wherein the material composites consist of several functional materials. They may be used, for example, in fuel cells. Liquid preparations which contain the catalyst material (catalyst inks) are used, inter alia, to produce the catalyst layers.
Furthermore, WO 97/23919 describes a process for producing membrane electrode assemblies, wherein linkage of the polymer electrolyte membrane, the electrode layers and the gas diffusion layers is performed continuously in a roller process.
U.S. Pat. No. 6,074,692 also describes a continuous process for simultaneously coating both sides of a polymer electrolyte membrane with catalyst layers, using appropriate catalyst inks, but without the application of gas distribution layers.
The electrochemical performance of membrane electrode assemblies depends, inter alia, on the thickness of the polymer electrode membrane. The thinner the membrane, the lower is its electrical resistance. Currently, membranes with thicknesses of 50 and 100 μm are used for membrane electrode assemblies. Since the membranes become ever more difficult to handle as they become thinner, they are sometimes supplied with a support film on one surface.
An object of the present invention is to provide a more reliable process with which polymer electrolyte membranes, in particular with thicknesses of less than 50 μm, can be processed to give membrane electrode assemblies.